1. Field of the Invention
The present invention relates to a slant-surface silicon wafer and a manufacturing method of its surface structure. More specifically, the invention relates to a slant-surface silicon wafer for a semiconductor device which has a slant surface of a particular structure and provides superior characteristics to an oxide film formed thereon, as well as to a manufacturing method of such a slant surface.
The invention also relates to a semiconductor device and, more specifically, to a high-performance semiconductor device which is constructed such that electrons move in a particular direction determined in accordance with the surface structure of a slant-surface silicon wafer.
2. Description of the Related Art
It is well known that the surface state of a silicon wafer influences defects and a breakdown voltage of an oxide film formed thereon. In this connection, various techniques have been proposed. Among those techniques are making the surface as clean as possible, and reconstructing a crystal surface structure of atomic-level dislocations. The latter techniques include repeated degassing in an ultrahigh vacuum (see Journal of Vacuum Science Technology, Vol. 7A, pp. 2901-2905, 1989), and current conduction heating in a particular direction (see Japanese Journal of Applied Physics, Vol. 31, pp. 1164-1169, 1992).
Conventionally, such a crystal surface structure is observed by using a scanning tunnel microscope (STM) or an analyzing device of reflection high energy electron diffraction (RHEED).
However, to observe a crystal surface structure by STM or RHEED, it is necessary to remove a native oxide film formed on a silicon surface: observation is conducted after a native oxide film is removed by subjecting a wafer to a high-temperature treatment in an ultrahigh vacuum of a pressure lower than 10.sup.-8 Pa. During this procedure, if the pressure becomes higher than 10.sup.-6 Pa, the surface is re-oxidized to make it difficult to observe the reconstructed surface.
In view of the above problems of the conventional techniques in reconstructing a silicon wafer crystal surface and confirming its reconstruction, the present inventors have found a method capable of easily reconstructing a crystal surface as well as easily recognizing the state of the reconstructed crystal surface. Further, the inventors have eagerly studied to realize a method of forming, on an industrial scale, silicon wafers having a crystal surface which enables formation of a high-quality thermal oxidation film that has only a small number of thermal-oxidation-induced stacking faults and has a high breakdown voltage while controlling the state of the crystal surface.
As a result, the inventors have found that by performing a high-temperature heat treatment in an ultrapure argon or hydrogen gas containing nitrogen at not more than 0.1 ppm, a native oxide film formed on a wafer surface can be decomposed as well as the wafer can be degassed, so that a surface can be reconstructed.
Further, in the conventional methods, since a wafer is exposed to the air from the end of a high-temperature heat treatment to the start of observation and analysis, a native oxide film is again formed thereon. Since the native oxide film is an insulator, in such analyzing methods as STM the native oxide film may render an analysis incorrect. In contrast, the inventors have found that by forming not only a slant surface that is inclined in a particular direction by a very small angle but also an atomic-level stepped structure, a reconstructed surface structure can be recognized easily, that is, it can be analyzed with an interatomic force microscope (AFM). The inventors have also found that the above technique enables estimation of the state of a reconstructed crystal surface of a wafer even if a native oxide film is formed at a certain thickness.
Turning now to the device aspect, when silicon wafers are used as substrates for various semiconductor devices, an oxide film is formed thereon as a kind of protection film for preventing dislocations and slips from occurring in the silicon crystal. An oxide film is required to be homogeneous and have a uniform flat surface. Properties of an oxide film are mainly influenced by the surface orientation and the surface state of a silicon wafer, though they are also influenced by the oxidizing method and apparatus. Therefore, an oxide film is formed by employing a given surface orientation, and performing careful cleaning, a heat treatment, and other treatments.
However, to manufacture a semiconductor device, it is not conventional at present that after the above processes a channel and a gate, for instance, are formed on a selected portion of a stepped crystal surface structure of a silicon wafer substrate.